Use to_boolean helper for backup force env

What does this MR do and why?

Fixes #358231 (closed)

Previously we explicitly required force=yes. Instead here we are using our Utils.to_boolean helper that accepts a few typical boolean values. This will make it more consistent with other boolean env vars.

While we could write tests to enumerate all possible values of skip I don't think it's really necessary. The fact that the tests still pass with the new boolean parser is sufficient.

How to set up and validate locally

1. force is only used on restore. So first we need to create a backup.

   $ bundle exec rake gitlab:backup:create
   ...
   2022-04-13 10:37:53 +1200 -- Backup 1649803057_2022_04_13_14.10.0-pre is done.

2. Restore from said backup with force set to false or nothing so we can see what happens. The prompt is what we use force to skip.

   $ bundle exec rake gitlab:backup:restore BACKUP=1649803057_2022_04_13_14.10.0-pre
   2022-04-13 10:39:35 +1200 -- Unpacking backup ... 
   2022-04-13 10:39:36 +1200 -- Unpacking backup ... done
   2022-04-13 10:39:36 +1200 -- Restoring database ... 
   2022-04-13 10:39:36 +1200 -- Be sure to stop Puma, Sidekiq, and any other process that
   connects to the database before proceeding. For Omnibus
   installs, see the following link for more information:
   https://docs.gitlab.com/ee/raketasks/backup_restore.html#restore-for-omnibus-gitlab-installations
   
   Before restoring the database, we will remove all existing
   tables to avoid future upgrade problems. Be aware that if you have
   custom tables in the GitLab database these tables and all data will be
   removed.
   
   Do you want to continue (yes/no)? 

3. Restore from the backup with force set. Notice that there's no longer a prompt before restoring the database.

   $ bundle exec rake gitlab:backup:restore BACKUP=1649803057_2022_04_13_14.10.0-pre force=1
   2022-04-13 10:42:26 +1200 -- Unpacking backup ... 
   2022-04-13 10:42:27 +1200 -- Unpacking backup ... done
   2022-04-13 10:42:27 +1200 -- Restoring database ... 
   2022-04-12 22:42:27 UTC -- Cleaning the database ... 
   2022-04-12 22:42:28 UTC -- done
   Restoring PostgreSQL database gitlabhq_development ... SET
   SET
   ...

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